I'm on SharePoint Online and I need to use the Rest API to query the Pages library.

Normally, I'd use GET, but since I'm filtering on a Managed Metadata field, I have to use POST instead. For some reason reason, however, I'm not able to get some fields even though I have it on my ViewFields (I'm specifically interested in EncodedAbsUrl or FileRef).

Are you trying to get the url of a File? Your query is getting the item, not the file, so you have to $expand=File and then you will have all the Files properties, including FileRef, FileLeafRef and EncodedAbsUrl